Borrowed this book awhile back its an interesting read, the celtic horse fittings were beautiful, and the Uffington White horse could be traced in the curvileanear design on some of the bronze fittings........
Searching for the Silures: The Iron Age in South-East Wales (Haunted Britain S.) by Raymond Howell (Paperback - 10 Nov 2006)
Silures, so it says on the web, maybe comes from the latin and means, as you say, People of the Rocks, because of the mountains of Wales.
Sulis (being the feminine).. Tombo did a marvellous essay on this one, but its no longer around.. Bob Stewart in The Waters of the Gap, gives the following Celtic explanation...
SUIL
Irish - it means an 'eye' or a hole
Gaelic - an 'eye' the centre ofa whirlpool, opening or orifice
Suileath - sharp sighted and knowing..
All of which tie in neatly with the sacred nature of the hot springs of Bath emerging from the underground.
If this is interpreted in a celtic fashion, than what you have is that this particular goddess of the underworld was also 'far-seeing' and 'knowing'. As a name it was also used by the romans because they also directly married celtic/roman god names (in their colonisations) within the environmental nature of the place.
Burl (not much help) says of of Silbury that it could refer to the 'the mound where willows grow' or could derive from 'Sigla' Old Norse boundary mark..... Tombo used the argument that Silbury was an 'eye' I think, so thats how he tied the Silbury/Suil up....